Since the discovery and elucidation of the structure of ferrocene in the early 1950s, metallocenes of transition metals such as iron, niobium, vanadium, tungsten, chromium, nickel, cobalt, manganese or ruthenium have been a continual source of intrigue for chemists and material scientists.
One traditional application of metallocenes was their used as catalysts in olefin polymerisation; reviewed, for example, by Younkin et al., Science, (2000) Vol. 287, pages 460–462 who also report recent developments of biscyclopentadienyl and monocylcopentadienyl metallocenes of “early” transition metal that are used as homogenous polymerisation catalysts.
As methods for producing more stable and active metallocenes were developed over the years, their use have expanded to other areas of application in which their electrochemistry had an important role, such as electronic applications including molecular switches, metal probes, molecular magnets, non-linear optics and, more commonly, as a mediator in enzyme electrodes.
Metallocenes such as ferrocenes are also currently undergoing a renaissance due to their increasing role in the rapidly growing area of material sciences. For example, ferrocene-containing materials have found widespread applications not only in catalysis but also biosensing, thermotrophic liquid crystals and non-linear optics (cf. A. Author, in Metallocenes. Synthesis Reactivity, Applications, Vol. 1, Chapter 1; Vol. 2, Chapter 11; A. Togni and R. L Halterman, Eds, Wiley, New York, 1998).
Iron in high oxidation states such as Fe(III) and Fe(IV) has also been found to be involved in biological processes. For example, a wide range of heme and non-heme iron-containing oxygenase enzymes were found to be involved in the mediation of oxygen atom transfer in biological systems. Additionally, dioxygen activation by metalloenzymes and biomimetic complexes has fuelled interest in high-valent metal complexes as models for biocatalysis. (cf. Chem. Rev. 96(1996) 2607–2624, 2625–265, 2659–2756, 2841–2887)
Due to their stability, ferrocene compounds are however typically based on the ferrocenium ion (Fc+) in which the iron atom is present as Fe (III), whereas the (Fc2+) species in which the iron is present in oxidation state (IV) have not been isolated because the stability of Fc2+ decreases significantly. The existence of such ferrocene derivatives was confirmed only electrochemically under the stringent conditions (in liquid SO2, −40° C. by Sharp & Bard (Inorg. Chem. Vol. 22, No. 19, 1983) or Gale et al (J. Organom. Chem. 199 (1980) C44–C46)
A similar instability was reported for other high oxidation state metallocenes earlier. For example, Wilson et al. (JACS 91, 758, (1969)) achieved the reversible oxidation of nickel from the oxidation state of (III) to (IV) in nickelocene using cyclic voltammetry in acetonitrile solution. However, it was necessary to use a process temperature of −40° C. in order to prevent the nickel (IV) species from rapidly decomposing. Likewise, Kuwana et al. (JACS, 82, 5811 (1960)) described a one-step, two electron oxidation of ruthenocene. In inert medium and at ambient temperature, the obtained ruthenocene dipositive ion decomposed over a period of about 10 hours.
Accordingly, there remains a need for stable metal complexes which can be used, either as model compounds or directly in practical applications, in the above mentioned areas such as biocatalysis or biosensing.